Dose rate correction in medium dose rate brachytherapy for carcinoma cervix.

Abstract

To establish the magnitude of brachytherapy dose reduction required for stage IIB and III carcinoma cervix patients treated by external radiation and medium dose rate (MDR) brachytherapy at a dose rate of 220+/-10 cGy/h at point A. In study-I, at the time of MDR brachytherapy application at a dose rate of 220+/-10 cGy/h at point A, patients received either 3060 cGy, a 12.5% dose reduction (MDR-12.5), or 2450 cGy, a 30% dose reduction (MDR-30), to point A and they were compared to a group of previously treated LDR patients who received 3500 cGy to point A at a dose rate of 55-65 cGy/h. Study-II was a prospective randomized trial and patients received either 2450 cGy, a 30% dose reduction (MDR-II (30)) or 2800 cGy, a 20% dose reduction (MDR-II (20)), at point A. Patients were evaluated for local control of disease and morbidity. In study-I the 5-year actuarial local control rate in the MDR-30 and MDR-12.5 groups was 71.7+/-10% and 70.5+/-10%, respectively, compared to 63.4+/-10% in the LDR group. However, the actuarial morbidity (all grades) in the MDR-12.5 group was 58.5+/-14% as against 34.9+/-9% in the LDR group (P < 0.05). Similarly, the grade III and IV morbidity also in the MDR-12.5 group was 12.5+/-9% as against 5.3+/-5% in the LDR group (P < 0.05). No statistically significant difference in morbidity was seen between the MDR-30 and LDR groups. In study-II the 3-year actuarial local control rate in the MDR-II (30) and MDR-II (20) groups was 66.6+/-10% and 74.8+/-9%, respectively. There was a significant correlation between the rectal BED received and the percentage of patients developing rectal morbidity. Only 10% of patients receiving a rectal BED of (100 < 120) Gy3 developed complication as against 62.5% of those receiving a rectal BED of (140 < 160) Gy3 (chi2 = 46.43; P < 0.001). We suggest that at a dose rate of 220+/-10 cGy/h at point A the brachytherapy dose reduction factor should be around 30%, as suggested by radiobiological data, to keep the morbidity as low as possible without compromising the local control rates.